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PubMed2003

Impulsive-motion model for computing the closing motion of mechanical heart-valve leaflets.

The speed of mechanical heart-valve leaflets is known to be an important quantity for predicting cavitation, yet no simple computational means exists for predicting the leaflet speed. In this study, a model for simulating the motion of heart-valve leaflets in rigid test systems is presented. The input for the simulations is the ventricular pressure trace, readily measured in heart-valve tests. The model is based upon an impulsive-motion approximation, wherein the motion within the system is produced by rapid acceleration at the boundary, e.g., by a moving piston. A set of quasisteady, linear equations for the pressure field that are decoupled from the leaflet equation of motion is derived. The pressure field and leaflet moment are computed without the need to treat moving boundaries. Model predictions of closing time compared favorably with those measured in a 1994 cavitation study. Computed values of leaflet tip speed were also compared with those of a previous study, at the same value of average pressure slope. The model values were in agreement with measured speeds, given the limitations of using the average pressure slope as a metric for comparison.

Bioprosthesis↗
PubMed1996

The Björk-Shiley Delrin tilting disc heart valve: historical perspective, design and need for scientific analyses after 25 years.

The Björk-Shiley Delrin (BSD) tilting disc heart valve was first used clinically in 1969. It is estimated that up to 24,000 BSD heart valves were implanted between 1969 and 1981, of which 7,000 may still be implanted in patients. The BSD valve provided a low-profile, quiet prosthesis with excellent hemodynamics. There have been only two reports of mechanical failure due to inlet strut fracture. Recent reports of wear of the Delrin disc combined with wear as observed on explanted BSD valves after 17-20 years in patients stimulated the formation of a Scientific Advisory Panel by Shiley to review the status of the BSD heart valve and make recommendations regarding patients living with BSD heart valves. The studies presented in this Supplement of The Journal of Heart Valve Disease summarize some of the scientific investigations conducted by Shiley and those recommended by this Panel. The material reviewed by the Panel included: (i) clinical publications on BSD heart valves; (ii) seventy-three explanted BSD heart valves returned to Shiley over the past 25 years; (iii) BSD valves subjected to accelerated wear tests; (iv) unimplanted BSD heart valves in Shiley's inventory; (v) in vitro studies of static leakage and pulsatile regurgitation of explanted and unimplanted "control' BSD heart valves; (vi) two Christiansen hip prostheses explanted from patients after five and 13 years; (vii) implant data for the BSD heart valve from the Karolinska Hospital, Stockholm, Sweden; and (viii) test reports for studies and analyses conducted by Shiley for the BSD heart valve. This article gives an overview of and introduction to the studies of the BSD heart valve presented in the following papers.

Heart Valve Prosthesis↗
PubMed1994

[Exercise capacity after heart valve replacement].

Exercise capacity following heart-valve replacement is dependent on how close to normal the artificial device can restore valve function, to what degree a preoperative impaired myocardial function and/or an increased pulmonary vascular resistance is normalized. The postoperative functional result can be determined by the subjective improvement of the patient, his functional capacity, exercise capacity, the central hemodynamics at rest and during exercise, and the systolic and diastolic function of the left and right ventricular myocardium. The subjective improvement of individual symptoms is obviously dependent on the degree of postoperative normalization of hemodynamics, especially of pressures in the pulmonary circulation. Subjective improvement can be objectified by comparing the functional capacities before and after surgery. Post-operative normalization of central hemodynamics and myocardial function does not happen immediately but within 3 to more than 12 months. A 12-month period can generally be expected in patients with mitral stenosis and increased pulmonary vascular resistance (> 400 dyn.sec.cm-5) prior to surgery. In patients with mitral and aortic regurgitation as well as with aortic stenosis and preoperative decrease of their left ventricular ejection fraction during exercise, continuous improvement of left ventricular pump function also may need up to 12 months. Physiological hemodynamic conditions generally are not restored by valve replacement. All prostheses are stenotic to forward blood flow because of the obstruction created by the narrowing of the valve area by sewing cuff and valve poppet. This may result in a hemodynamically important stenosis, especially after atrio-ventricular valve implantation, and may limit subjective and functional improvement. Exercise capacity after aortic valve replacement depends mainly on whether or not myocardial damage persists postoperatively. A workload of 1.5 w/kg body weight (BW) has been performed by 100% of patients aged 45 to 55 years with prostheses implanted for aortic stenosis. The significant lower exercise capacity all patients with valve replacement for aortic regurgitation have experienced (0.4 w/kg BW) indicates that a substantial number of these patients has irreversible myocardial damage prior to surgery. The workload experienced by patients with mitral valve prostheses varies between 0.4 and 2.0 w/kg BW (mitral stenosis) and 0.3-2.3 w/kg BW (mitral regurgitation), respectively. To objectify the functional result of heart-valve replacement, hemodynamic-metabolic measurements of functional improvement, determination of left, eventually also of right-ventricular function by echocardiography and additional invasive measurements of the central hemodynamics and myocardial pump function parameters at rest and during exercise might be necessary.

Adult↗
PubMed1976

Management of the patient with a prosthetic heart valve.

Approximately 20,000 heart valve prostheses are inserted yearly in the United States. Even after successful heart operations, the patients who receive them cannot be regarded as healthy individuals but are a special group with special problems who need close medical attention for the rest of their lives. They are susceptible to many unusual complications because of their implanted foreign body, and it is a challenge to all physicians in contact with them to be aware of their peculiar problems in order to prevent complications if possible and to treat them immediately if they occur. General therapy, surgical complications, infection, and mechanical problems are reviewed, with means for management outlined. These difficulties can be dealt with only by careful follow-up and well-coordinated teamwork between the family physician and the institution where the operation was performed.

Activities of Daily Living↗
PubMed1976

Tissue culture, protein and collagen synthesis in antibiotic sterilized canine heart valves.

Viability of canine heart valve leaflet fibroblasts was assessed after varying periods of sterilization and storage in antibiotic-nutrient solution. Tissue culture and assessment of protein and collagen synthesis showed that tissue obtained under optimal conditions rarely retains viability beyond 3 weeks in antibiotic-nutrient solution and is severely impaired after 2 weeks. This casts serious doubts on viability in current clinical homograft valve practice.

Animals↗
PubMed1987

[Anticoagulation with low molecular-weight heparin in patients with prosthetic heart valve replacements].

In six patients with prosthetic heart valve replacement anticoagulation was performed with low molecular weight heparin for 4-58 weeks. The treatment was indicated because of one or more severe cerebral or gastrointestinal bleeding complications during therapy with oral anticoagulants or conventional heparin. The dose of the low molecular weight heparin ranged individually from 2,500 to 12,000 units once a day subcutaneously and was adjusted on the basis of the general bleeding tendency of the patient and the specific anticoagulant effect on factor Xa. Under this treatment no heart valve thrombosis occurred. Two minor bleeding complications were observed in two patients. All patients suffered previously from severe bleeding complications with conventional anticoagulants. One additional patient, who had been treated one year earlier with the low molecular weight heparin, again experienced embolism during treatment with only 1 X 5,000 anti factor Xa units per day. We conclude that anticoagulation with low molecular weight heparin may be recommended for patients with prosthetic heart valve replacement and severe bleeding problems with conventional anticoagulants. In patients with recurrent embolism higher doses should be administered.

Adult↗
PubMed1990

Estimation of shear stress-related blood damage in heart valve prostheses--in vitro comparison of 25 aortic valves.

The hemodynamics of heart valve prostheses can be reproducibly investigated in vitro within circulatory mock loops. By measuring the downstream velocity and shear stress fields the shear stresses which are clinically responsible for damage to platelets and red blood cells can be determined. The mechanisms of damage and the effects of shear stresses on blood corpuscles were investigated by Wurzinger et al. at the Aerodynamics Institute of the RWTH Aachen. In the present study, the above data are incorporated into a mathematical correlation, which serves as a basic model for the estimation of blood damage. This mathematical model was applied to in vitro investigations of 25 different aortic valve prostheses. The results were compared to clinical findings. In most cases agreement was good, indicating that this model may be directly applied to the clinical situation. This new method facilitates the estimation of clinically expected blood damage from in vitro measurements. It may be useful for the development and evaluation of new valve prostheses. By comparative evaluation of different valve types it also provides additional information to help the implanting surgeon select the optimum valve for his patient.

Aortic Valve↗
PubMedDate not supplied

Mechanical heart valve prostheses: identification and evaluation (erratum).

Mechanical heart value prostheses have been in use since the 1950s. Many prostheses have been used for a while and then discontinued. Today, there are a large number and variety of prostheses in use and an even larger variety that are in place in patients. These may be explanted at any time for a number of reasons. It is essential for the practicing pathologist to be able to identify the prosthesis and be aware of some of its reported complications and modes of failure. This article, and a second one on bioprosthetic heart valves, is designed as a ready reference guide to heart valve prostheses, their important identifying features, their common complications, and modes of failure. It should help in the accurate identification of explanted prosthetic valves and more definitive reports. This accuracy of identification as well as tracking of abnormalities noted will, we hope, permit the identification of new failure modes and the recording of causes of failure of new (or even modified) prosthetic heart valves.

Heart Valve Prosthesis↗
PubMed1976

Comparative study of in vitro flow characteristics between a human aortic valve and a designed aortic valve and six corresponding types of prosthetic heart valves.

A model of a pulsatile cardiovascular system is described including an electronic control, a hydraulic driving unit, a volume storage, a peripheral resistance and measuring equipment. Six different artificial heart valves, a designed aortic valve (Aachen pockett valve II) and a human aortic valve have been tested in the outflow position of the pump. Three of them are presently being used in clinical applications. Measurements of pressure and volume flow have been obtained for different frequencies n, pump volumes Vp and time ratios between pressure and suction phase. From the experimental results a comparative evaluation was deducted for different working conditions. In comparison to natural human aortic valve (NK) large volume flow is verified by the Aachen pocket valve II (AT), the Gott-Dagget value (GD), the Starr-Edwards valve (SE) and the Lillehei-Kaster valve (LK). The Gott-Daggett valve has the smallest back flow volume. The Smeloff-Cutter and in particular the Wada-Cutter valve have the largest regurgitation. This is to be expected since the Cutter valves are purposely so designed that they do not close tightly in order to avoid blood sedimentation. The human aortic valve, the Gott-Daggett valve and the Aachen pocket valve demonstrate the most favourable opening-closing characteristics. The human aortic valve achieves a negligible pressure loss, followed by the Björk-Shiley and the Lillehei-Kaster valve.

Adult↗
PubMed1995

On the origin of cerebrovascular microemboli associated with prosthetic heart valves.

Application of transcranial Doppler ultrasonography to asymptomatic prosthetic heart valve patients can result in the detection of transient high intensity signals, similar to those induced by the passage of emboli. However, the origin of these signals is unknown. An in vitro study has been undertaken to investigate the capacity of prosthetic heart valves to generate high intensity Doppler signals in the absence of blood. A pulse duplicator, filled with a seeded saline solution, was used to function prosthetic heart valves under mock-physiological conditions. A Björk-Shiley Monostrut valve was mounted in the aortic port while a tri-leaflet control valve was fixed in the mitral port. At stations upstream and downstream from the Björk-Shiley valve, flow was monitored using pulsed wave Doppler ultrasound (Nicolet TC-2000, 2 MHz probe). The effect of damping the harsh closure of the mechanical valve was investigated by applying a thin layer of soft adhesive tape between the valve occluder and outer ring. For all valve configurations, transient high intensity Doppler signals, characteristic of microemboli and similar to those observed in clinical studies of prosthetic heart valve patients, were detected downstream from the aortic port. The number of microemboli signals did not change significantly between sites at 20 cm and 40 cm downstream from the aortic valve. Damping the Björk-Shiley valve closure greatly reduced (by 80%) the number of microemboli signals detected. It is concluded that Doppler microemboli signals can be generated by prosthetic heart valves while functioning in the absence of the formed elements of blood, and that the number of microemboli signals produced depends upon the rate of energy dissipation at valve closure.

Aortic Valve↗
PubMed2006

Prevention of device-related tissue damage during percutaneous deployment of tissue-engineered heart valves.

BACKGROUND: Endovascular application of pulmonary heart valves has been recently introduced clinically. A tissue-engineering approach was pursued to overcome the current limitations of bovine jugular vein valves (degeneration and limited longevity). However, deployment of the delicate tissue-engineered valves resulted in severe tissue damage. Therefore the objective of this study was to prevent tissue damage during the folding and deployment maneuver. MATERIAL AND METHODS: Porcine pulmonary heart valves, small intestinal submucosa, and ovine carotid arteries were obtained from a slaughterhouse. After dissection and antimicrobial incubation, the valves were trimmed (removal of sinus and most of the muscular ring) to fit into the deployment catheter. The inside (in-stent group, n = 6) or outside (out-stent group, n = 6) of a nitinol stent was covered by an acellular small intestinal submucosa, and the valves were sutured into the stent. The valves were folded, tested for placement in the deployment catheter, and decellularized enzymatically. Myofibroblasts were obtained from carotid artery segments and seeded onto the scaffolds. The seeded constructs were placed in a dynamic bioreactor system and cultured for 16 consecutive days. After endothelial cell seeding, the constructs were folded, deployed, and processed for histology and surface electron microscopy. RESULTS: The valves opened and closed competently throughout the entire dynamic culture. Surface electron microscopy revealed an almost completely preserved tissue in the in-stent group. Stents covered with small intestinal submucosa on the outside, however, showed severe damage. CONCLUSION: This study demonstrates that small intestinal submucosa covering of the inside of a pulmonary valved stent can prevent stent strut-related tissue damage.

Animals↗
PubMed1989

Development of a ceramic heart valve.

A durable and thromboresistant ceramic heart valve comprised of a single crystal alumina disk and titanium nitride (TiN) valve ring has been developed. Blood compatibility was examined by scanning electron microscopy (SEM) examinations of the valves implanted in sheep for 35 (#1), 26 (#2), 20 (#3), 23 (#4), and 26 (#5) days. The single crystal alumina and TiN surfaces were free of platelet aggregation or fibrin networks, except for some depositions of fibrin and platelets on the outflow TiN ring in #3, and isolated red cells on the outflow TiN ring in #5. Durability testing under high pressure (1750 mmHg = 233 KPa) pulsatile conditions showed that the safety factor of the ceramic valve was more than seven times greater than anticipated. The ceramic valve is promising as an artificial heart valve.

Animals↗
PubMed2000

Functional living trileaflet heart valves grown in vitro.

BACKGROUND: Previous tissue engineering approaches to create heart valves have been limited by the structural immaturity and mechanical properties of the valve constructs. This study used an in vitro pulse duplicator system to provide a biomimetic environment during tissue formation to yield more mature implantable heart valves derived from autologous tissue. METHODS AND RESULTS: Trileaflet heart valves were fabricated from novel bioabsorbable polymers and sequentially seeded with autologous ovine myofibroblasts and endothelial cells. The constructs were grown for 14 days in a pulse duplicator in vitro system under gradually increasing flow and pressure conditions. By use of cardiopulmonary bypass, the native pulmonary leaflets were resected, and the valve constructs were implanted into 6 lambs (weight 19+/-2.8 kg). All animals had uneventful postoperative courses, and the valves were explanted at 1 day and at 4, 6, 8, 16, and 20 weeks. Echocardiography demonstrated mobile functioning leaflets without stenosis, thrombus, or aneurysm up to 20 weeks. Histology (16 and 20 weeks) showed uniform layered cuspal tissue with endothelium. Environmental scanning electron microscopy revealed a confluent smooth valvular surface. Mechanical properties were comparable to those of native tissue at 20 weeks. Complete degradation of the polymers occurred by 8 weeks. Extracellular matrix content (collagen, glycosaminoglycans, and elastin) and DNA content increased to levels of native tissue and higher at 20 weeks. CONCLUSIONS: This study demonstrates in vitro generation of implantable complete living heart valves based on a biomimetic flow culture system. These autologous tissue-engineered valves functioned up to 5 months and resembled normal heart valves in microstructure, mechanical properties, and extracellular matrix formation.

Absorbable Implants↗
PubMed1990

Artificial heart valves.

This overview of heart valve prostheses is based on a current review of clinical reports and focuses on the major complications that characterize long-term valve performance: thromboembolism, thrombosis, anti-coagulant-related bleeding, and structural failure.

Bioprosthesis↗
PubMed2006

Prosthetic heart valves: Objective Performance Criteria versus randomized clinical trial.

The current Food and Drug Administration (FDA) heart valve guidance document uses an objective performance criteria (OPC) methodology to evaluate the clinical performance of prosthetic heart valves. OPC are essentially historical controls, but they have turned out to be an adequate, and perhaps optimal, study design in this situation. Heart valves have a simple open-and-close mechanism, device effectiveness is easy to document, and the common complications (thromboembolism, thrombosis, bleeding, leak, and infection) are well known and easily detected. Thus, randomized clinical trials (RCTs) have not been deemed necessary for the regulatory approval of prosthetic heart valves. The OPC are derived from the average complication rates of all approved heart valves. Studies based on OPC have been shown to work well; many different valve models have gained FDA market approval based on this methodology. Although heart valve RCTs are not required by the FDA, they have been done to compare valves or treatment regimens after approval. Recently, the Artificial Valve Endocarditis Reduction Trial (AVERT) was designed to compare a new Silzone sewing ring, designed to reduce infection, with the Standard sewing ring on a St. Jude Medical heart valve. This was the largest heart valve RCT ever proposed (4,400 valve patients, followed for as long as 4 years), but it was stopped prematurely because of a high leak rate associated with the Silzone valve. Examining the results showed that a much smaller, OPC-based study with 800 patient-years would have been sufficient to disclose this complication of the Silzone valve.

Bayes Theorem↗
PubMed1994

Thromboembolic and bleeding complications in patients with mechanical heart valve prostheses.

BACKGROUND: Patients with mechanical heart valve prostheses may experience valve thrombosis and subsequent systemic embolism for which they are treated with oral anticoagulant therapy. It is essential to know reliable estimates of the risks and benefits of this therapy in order to answer a number of clinical questions rationally. We sought to obtain more precise estimates of the risks and benefits by combining the data from individual studies by using meta-analysis. METHODS AND RESULTS: We searched for studies in which the incidences were reported of embolic or bleeding complications in patients with mechanical heart valve prostheses. They were collected from the Medline and Current Contents database and by cross-references between 1970 and 1992. Since most studies vary greatly in many respects, we used a number of inclusion criteria, thus selecting comparable studies of acceptable quality only. The influence of antithrombotic therapy, valve position, and valve type was analyzed by univariate and by multivariate analysis with Poisson regression techniques. Forty-six studies were found, including 13,088 patients studied for 53,647 patient-years. We found an incidence of major embolism in the absence of antithrombotic therapy of 4 per 100 patient-years. With antiplatelet therapy this risk was 2.2 per 100 patient-years, and with coumarin therapy it was reduced to 1 per 100 patient-years. This risk varied with the type and the site of the prosthesis. A prosthesis in mitral position increased the risk almost twice as compared with the aortic position. Tilting disc valves and bileaflet valves showed a lower incidence of major embolism than caged ball valves. An incidence of major bleeding was found in patients treated with coumarin derivatives of 1.4 per 100 patient-years. The incidence of bleeding became significantly higher with the addition of antiplatelet therapy, although this did not decrease the risk of thromboembolism any further. CONCLUSIONS: These data provide a reference for future studies and give adequate risk estimates for clinical decision making.

Anticoagulants↗
PubMed1985

Metal wear in Lillehei-Kaster heart valve prostheses.

Ten Lillehei-Kaster heart valve prostheses, in situ for up to 10 years and recovered at surgery or necropsy, were examined by light and scanning electron microscopy. All showed metal wear on the luminal aspect of their struts. The volume of wear related to the duration a prosthesis had been in situ. The worn metal showed distinct, transverse surface corrugations, which became more obvious with time. Aortic prostheses wore more and faster than mitral ones. One strut usually showed more wear than the other, a change likely due to specific manufacturing methods. It is believed that the pattern of wear is caused by a velocity-controlled stick-slip abrasive wear process, resulting from an interaction between the edge of the moving pyrolytic carbon disc, the struts' titanium surface, and the protein coat covering that surface. None of the patients had prosthesis dysfunction attributable to metal wear. Disc escape seems unlikely considering the degree of wear observed after 10 years. Furthermore, the surface corrugations did not appear to cause disc sticking or other problems. However, clinicians might consider monitoring patients who have borne these prostheses for greater than 10 years.

Aortic Valve↗
PubMed1991

Cavitation potential of mechanical heart valve prostheses.

Just like technical check valves, the function of mechanical heart valve prostheses may presumably also lead to cavitation effects during valve closure. Due to the waterhammer effect, cavitation may primarily occur in the mitral position leading to high mechanical loading of the valve itself and of corpuscular blood elements. Ten different types of commercial mechanical heart valves were investigated in the mitral position of a pulsatile mock loop, to detect cavitation thresholds under physiologically similar conditions by cinematographic techniques. Almost all these valve prostheses show cavitation up to a ventricular pressure gradient of 5000 mmHg/s. The threshold depends on valve type and size and is sometimes within the physiological range below 2000 mmHg/s. Visible cavitation bubbles with a diameter of up to 1.8 mm and a collapse time of less than 0.1 ms suggest that vapour cavitation may play an important role for material and blood damage in mechanical heart valve prostheses.

Coronary Circulation↗